Team demonstrates molecular-scale memory

Portland, Ore. -- As chip dimensions shrink, picometer variability among nanoscale dimensions and the uneven distribution of dopants stand in the way of further miniaturization. Use of a precisely designed organic molecule as the memory storage element could provide one solution, because the molecules could be mass-produced to be identical.

Recently, the IBM Research Laboratory (Zurich, Switzerland) demonstrated one such molecule, which it claims can be electrically programmed to store a bit in two bistable states.

"For further miniaturization, variability is becoming an issue: Dimensions, and thus electrical characteristics, may vary from transistor to transistor," said Heike Riel, lead researcher at IBM's Zurich Research Laboratory. "However, molecules have the advantage that they are fabricated by chemical synthesis and hence can all be made identical."

To get a leg up on molecular-scale memory devices, IBM demonstrated how identical molecules could solve the miniaturization problem for future memory chips beyond the self-proclaimed "end" of the International Technology Roadmap of Semiconductors (ITRS), expected in about 16 years. By showing that a single organic molecule (bipyridyl-dinitro oligophenylene-ethynylene dithiol, or BPDN-DT), com- posed of just a few atoms of nitrogen and oxygen, can store a bit of information in a bistable state, IBM hopes to energize EEs worldwide to pursue molecular-scale memory devices.

"The goal of our experiment was to demonstrate that the switching effect observed in the molecule is truly of molecular origin," said Riel, who received last year's Applied Physics Award of the Swiss Physical Society for her work on organic LEDs. "In general, our goal is to investigate and understand the charge-carrier transport through molecular junctions--that is, to correlate the materials chemistry and function of various organic molecules."

The experimentThe team, which included fellow IBM researcher Emanuel Lörtscher, connected two gold electrodes to a single organic molecule that had been custom-designed for the work by Jim Tour and his group at Rice University (Houston, Texas).

Starting with the molecule in the "off" state, a write pulse of 1.6 volts switched the molecule to its "on" state, which was read out using a 1.1-V pulse. The molecule was then switched back to the "off" state by an erase pulse of –1.6 V. Both the "off" (binary 0) and the "on" (binary 1) states were stable and nondestructive during readouts. The researchers carefully characterized more than 500 stable cycles with switching times in the microsecond range and memory retention of about 30 seconds (in contrast, DRAM has only milliseconds of memory retention before needing to be refreshed).

IBM used the mechanically controllable break-junction (MCBJ) technique to connect gold electrodes to the single organic molecule. MCBJ was invented in 1985 at the National Bureau of Standards (now the National Institute of Standards and Technology), which used the technique to elucidate the functioning of tunneling junctions. In the 1990s, the University of Leiden used MCBJ to investigate metal-to-metal junctions.

In resurrecting the MCBJ technique for the single-molecule memory prototype, Riel said, the IBM team "automated and refined it to perform our statistical measurements, which was crucial for investigating single molecules."

The MCBJ technique stretches a thin film of metal (gold in the IBM experiment) by flexing its substrate. The metal film is fabricated in such a way that it harbors a small bridge. Flexing the substrate elongates the molecules of the bridge until only a single atom remains. At that point the bridge breaks, leaving two closely spaced electrodes.

The IBM researchers then adjusted the distance between the electrodes with sub-picometer accuracy by flexing the substrate further. Next the researchers deposited a monolayer of organic memory molecules, just one of which would fit into the gap sized to accommodate the selected organic molecule (typically, just 1.5 nanometers in length). Using this method, the team of researchers was able to insert a single organic molecule between the electrodes with about 50 percent yield.

The researchers tested two molecules--BPDN-DT and a reference molecule of bipyridyl oligophenylene- ethy nylene dithiol (BPDT), which did not switch--to demonstrate that it was indeed the construction of its BPDN-DT molecule, and not tunneling between the electrodes, that had induced the switching.

For the future, the IBM team hopes to increase its yield from 50 percent, then begin fully characterizing the switching phenomenon in hopes of elucidating the mechanism causing it.

"Possible reasons for switching could be conformational change, trapping of charges on the molecule or tilting," said Riel.

Higher temps a goalThe team also aims to increase the operating temperature beyond the frosty 100 Kelvin (–279°F or –173°C) at which the current molecular device was operated. Switching can be observed at room temperature today, but the junction becomes unstable after a few switching cycles because of metal migration during high-electric fields from the increased mobility of the gold electrodes' atoms at room temperature.

Future experiments will also characterize the bit retention times and switching dynamics as a function of temperature.